What is Public Interest?

Legal Definition
Public interest, according to the Random House Dictionary, is "1. the welfare or well-being of the general public; commonwealth. 2. appeal or relevance to the general populace: a news story of public interest."

Economist Lok Sang Ho in his Public Policy and the Public Interest (Routledge, 2012, published 2011) argues that the public interest must be assessed impartially and, therefore, defines the public interest as the "ex ante welfare of the representative individual." Under a thought experiment, by assuming that there is an equal chance for one to be anyone in society and, thus, could benefit or suffer from a change, the public interest is by definition enhanced whenever that change is preferred to the status quo ex ante. This approach is "ex ante", in the sense that the change is not evaluated after the fact but assessed before the fact without knowing whether one would actually benefit or suffer from it.

This approach follows the "veil of ignorance" approach, which was first proposed by John Harsanyi but popularized by John Rawls in his 1971 Theory of Justice. Historically, however, the approach can be traced to John Stuart Mill, who, in his letter to George Grote, explained that "human happiness, even one's own, is in general more successfully pursued by acting on general rules, than by measuring the consequences of each act; and this is still more the case with the general happiness, since any other plan would not only leave everybody uncertain what to expect, but would involve perpetual quarrelling..."

The Institute of Chartered Accountants in England and Wales argues that applying a detailed definition is likely to result in unintended consequences, in Acting in the Public Interest(2012). Instead, each circumstance needs to be assessed based on criteria such as the relevant public, wants, and constraints. The key to assessing any public interest decision is transparency of the decision-making process, including balancing competing interests.
-- Wikipedia
Legal Definition
The welfare of the public as compared to the welfare of a private individual or company. All of society has a stake in this interest and the government recognises the promotion of and protection of the general public. This term is vague but the government will only let the public know what is in the publics’ best interest. It won’t release information that could cause riots and upheaval in the nation.